in order to not to get much frustrated by 1 month of constant and uniform cloudy sky (night and day) I decided to invest in studying a bit.
In particular I am interested in sub-exposures.
tools.sharpcap.co.uk is a great tool, and I wanted to reverse engineer it to put the logic in a spreadsheet (that I can use offline).
What I found by examining the javascript embedded in the web page is the formula attached to this topic as an image ("," is actualy decimal point "." in the equation, px is the pixel size in micron, skyMag is the value reported by Lightpollutionmap.info's SQM, f_sky is the "flux", electrons per pixel per second).
If I didn't make any mistake (apparently I didn't as the equation provides results matching those ones calculated by tools.sharpcap.co.uk), I would like to ask some questions (to some expert or even the sharpcap's creator Dr. Robin Glover):
- Where the 3.837 factor comes from ? and why 550 instead of the full 300 (nm) visible bandwidth spectrum ?
- The skyMag (SQM) is usually calculated at Zenith... but it is not unusual to try to photograph a DSO which is low on the horizon (e.g. in this period spring galaxies are too small for my gear, then 1 month ago - the last clear night - I tried the out-of-season Rosette and Heart); any hint on how to get a good estimation of the skyMag in the case of an object not much high in the sky ?
- I would like to extrapolate a similar equation for the photons' flux converted into e- (e- per pixel per second) in the case of a faint deep sky object (I would say something like: S_dso=(3.837*BW/550)*(QE/100)*(px/F_ratio)^2*(10^(-0.4*Mag_dso)), where Mag_dso is the value that I can get from astro catalogs, Stellarium, etc.)
Alvise